1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to a copier, a printer, a facsimile or any other electrophotographic image forming apparatus that transfers a toner image on a recording sheet and thereafter applies heat to the recording sheet for fixation of the toner image thereon.
2. Description of Related Art
An image forming apparatus that forms a toner image on a recording sheet by an electrophotographic process comprises a fixing device configured to apply heat to the recording sheet for fixation of the toner image thereon. It is known that when the fixing device is driven, heat is generated, which causes emanation of a volatile material from rollers and other parts of the fixing device. Further, recently, it has been found that ultrafine particles (particles with diameters not more than about 1000 nm) are generated from the volatile material and discharged from the image forming apparatus.
The ultrafine particles discharged from the image forming apparatus are a tiny amount, and the tiny amount of ultrafine particles themselves cause no particular damage to human health or environments. However, with increasing awareness of environmental issues, it is desirable that such apparatuses are put under emission control to prevent even a tiny amount of emission. Also, it is expected that toner particles become still finer, and it would be necessary to control emission of such toner particles from image forming apparatuses.
For example, Japanese Patent Laid-Open Publication No. 2007-171921 discloses a technical skill to trap exhaust emissions efficiently inside an image forming apparatus. In an image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 2007-171921, in order to prevent volatile organic compounds generated inside the image forming apparatus from being discharged therefrom, while air is circulated around a fixing device, a filter provided in the circulation path traps the volatile organic compounds.
Japanese Patent Laid-Open Publication No. 2009-151075 discloses an image forming apparatus wherein a circulation path is switchable between an inner circulation path and an outer circulation path for the purpose of dew condensation prevention. The circulation path is switched to the inner circulation path or the outer circulation path, depending on the temperature and the humidity inside and outside the image forming apparatus, and thereby, condensation on an exposure device and contamination of a glass surface by a filter are prevented.
Japanese Patent Laid-Open Publication No. H3-267954 discloses an electrophotographic apparatus wherein while exhaust air from a suction section for sucking and conveying a sheet is circulated inside the apparatus, a filter traps toner, ozone and other matters.
However, as Japanese Patent Laid-Open Publication No. 2007-171921 discusses, circulating the air that has once passed through a filter inside an apparatus is circulating heated air inside an apparatus, which may cause a rise in the temperature inside the apparatus. Also, as Japanese Patent Laid-Open Publication No. 2009-151075 discusses, in a case where the switch of the circulation path between the inner circulation path and the outer circulation path depends on the temperature and the humidity inside and outside the apparatus, relatively large particles such as dust and toner can be trapped.
As concerns ultrafine particles, however, if the discharge speed from the apparatus is too high, the ultrafine particles may pass through the filter and be discharged from the apparatus. Circulating the exhaust air from the suction section inside the apparatus as taught in Japanese Patent Laid-Open Publication No. H3-267954 may cause a problem of a temperature rise inside the apparatus, as in the apparatus disclosed in Japanese Patent Laid-Open Publication No. 2007-171921.
Generally, by heightening the efficiency of removing fine particles by a filter, the trapping efficiency is improved, but the pressure loss becomes higher. In order to address the problem, a greater power source (fan) is necessary, which causes problems in terms of cost, space, energy-saving and noise. With regard to a sheet suction section, lowering the flow rate too much impairs stable sheet conveyance and separation between sheets. Also, with regard to trapping of ultrafine particles, a high flow rate causes ultrafine particles to slip through a filter, which results in a decrease in the trapping efficiency. Thus, in terms of particle trapping efficiency, it is desired that the flow rate is minimized.